Central Somatosensory Pathways Flashcards
What is the general route of mechanosensory afferent fibres?
Fast and myelinated Enters spinal cord or brain stem Ascend in white matter dorsal columns Makes local connections in spinal cord Very long
What is the topography of dorsal white matter in the spinal cord?
Lower levels tend to be close to midline
- Sacral and lumbar
Thoracic level more lateral
Cervical level most lateral
Where is the first synapse for ascending fibres?
Dorsal column nuclei in brain stem
What are the other names for the dorsal column nuclei?
Gracile and cuneate nucleus
Describe the main tactile mechanoreceptive pathway
Three neuron sequence to reach somatosensory cortex
Cell bodies of sensory fibres in dorsal root ganglia or trigeminal ganglion
- Enter dorsal column in spinal cord
- Terminate in dorsal column nuclei in caudal medulla
Decussation in caudal medulla
- Arcuate fibres cross to other side
next neurons ascend in flat bundle of fibres = medial lemniscus
- Loops around brain stem structures as it moves up
Next synapse in ventral posterolateral nucleus in thalamus
- Topography maintained
Third neuron = thalamic neuron
- Projects to somatosensory cortex
What is the main tactile mechanoreceptive pathway called?
Dorsal column medial lemniscus pathway
Where does a lot of the proprioceptive information from muscle end up in the brain?
Cerebellum, via spinocerebellar tracts
What functional distinction in terms of information does the central sulcus represent?
Forward from central sulcus = action
Behind from central sulcus = sensation and perception
Where is the somatic sensory cortex located?
Postcentral gyrus
What is the Brodmann’s area for the primary somatic sensory cortex?
SI divided into (moving away from central sulcus):
- 3a
- 3b
- 1
- 2
What part of the thalamus does face sensory information from the trigeminal nerve go?
Ventral posterior medial nucleus (VPM)
What part of the thalamus does spinal sensory information go?
Ventral posterior lateral nucleus (VPL)
What does the secondary somatic sensory cortex do?
Gets very little thalamic information
More downstream processing
Describe the topography of the primary somatic sensory cortex
Parts of body that are near each other are generally represented close to each other
Map is replicated across four Brodmann areas
Huge magnification of some areas of body reflecting density of mechanoreceptors from that area
Is the secondary somatic sensory cortex also topographically organised?
Yes, but not based on thalamic input
What do the posterior parietal cortices do?
Represent left and right parts of body
Which areas receiving somatic sensory information may not be as strictly somatotopic?
SII
2 posterior parietal cortices
How are the two sides of the body represented in the thalamus?
Represented areas mirror reflections
Is there any segregation of different sensory receptor fields?
Yes, there’s some degree
Eg: both slowly adapting and rapidly adapting receptor fields clustered
Describe the connections within the brain that establish functional heirarchies
Thalamus
Main destination of thalamic information are 3b
Area 3b shares it with other areas
Cortical areas start to segregate features > brought back together to perceive single object - don’t know how or where
More higher order information gives information about things including:
- Grip
- Force
- Identity of object
Secondary somatosensory cortex
- Sends information onto hippocampus and amygdala
Parietal areas 5 and 7 also receive information from primary somatosensory cortex
- Sends onto motor and premotor cortical areas
What is the hippocampus’ role in somatosensing?
Role in spatial memory
What is the amygdala’s role in somatosensing?
Involved in emotion - unpleasant touches
What kind of information might the parietal areas 5 and 7 be sending to the motor and premotor cortical areas?
More synthetic information to help with motor control
What are the three key points of cortical organisation?
Topography
Magnification
Plasticity
What happens to the area representing a finger after it has been amputated?
Territory taken over by remaining digits
Does plasticity occur at all points of the dorsal column medial lemniscus pathway?
Yes, evidence suggests so
What happens when regenerating nerves grow back to the wrong place?
Mislocalisation
No adaptation
What is a phantom limb?
After a part of body has been lost, normal outcome > person still perceives missing part still present
Can be
- Complete
- Incomplete
Why might a phantom limb happen?
No reason to change information in brain because it’s not being updated - brain doesn’t know information HAS been changed
When might a phantom limb cause problems?
When part was lost because of trauma
Patient feels excruciating pain, as this was the last sensation the brain perceived to be in that area, and it can’t be updated
